This invention, broadly, relates to alternators. In a more specific aspect, the invention pertains to inductor alternators.
Traditional alternators, that is, those non-inductor alternators employed in machines such as automobiles, provide either moving conductors which cut through a stationary magnetic field (rotating armature alternators), or a moving magnetic field, which passes over stationary conductors (rotating field alternators). In either case an induced electromotive force (emf) is produced in the conductors or in armature windings. Hence, traditional alternators include a field, an armature or output winding, and motion between the two.
In a rotating armature alternator an essentially stationary magnetic field is provided, usually by current in field coils. The armature, acting as a rotor, revolves in this field, cutting lines of magnetic flux, thus inducing in the armature windings an emf capable of driving a current. In a rotating field alternator, the arrangement is reversed. The armature coils are fixed on the stator, and the field is produced in the rotor. As the rotor rotates, the flux through the armature windings changes, and an emf is induced on the essentially fixed armature windings. Generally speaking, traditional alternators require electrical connections between the moving rotor and the fixed structure of the alternator. In the case of rotating field alternators, this connection is needed to supply the field coils with current to produce the magnetic field; in the case of rotating armature alternators, it is required to carry the output current. The usual means of achieving an electrical connection with the rotor in an alternator is to mount brushes on the fixed structure of the alternator and hold them in contact with sliprings on the rotor. Brushes and sliprings have a number of disadvantages: they are subject to constant wear, increasing the maintenance requirements of alternators; the wear and friction make them a frequent point of failure in alternators; they are often a limiting factor in the rotational speed of alternators.
In conventional inductor alternators, neither the field nor the output windings rotate: both the field coils (or permanent magnetic material) and the output windings are mounted on the stator. An emf is induced in the output windings by periodic changes in the magnetic flux through the windings caused by corresponding changes in the reluctance of the magnetic subcircuit carrying the magnetic flux through the windings. The reluctance is governed by the shape of the rotor relative to the stator. As the rotor moves it creates, due to this shape, changes or pulsations in the reluctance of local sub-circuits of the overall magnetic circuit carrying the flux. The rotor serves only as a component of the magnetic circuit or flux path of the alternator and so carries no coils or windings. Hence, there is no need for electrical connections with the rotor, and such devices as brushes and sliprings are eliminated.
Broadly, inductor alternators can be placed in two categories. If the direction of the magnetic flux through each section of the rotor remains the same as the rotor spins, the machine is termed a homopolar inductor alternator. If the direction of flux changes in sections of the rotor as it rotates, the machine is a heteropolar inductor alternator. The patent art, insofar as I have been able to determine is exemplified by timing devices such as U.S. Pat. No. 3,634,743, and relays such as U.S. Pat. No. 3,036,248. In the absence of analogous or pertinent patent art, I refer to such textbooks as Alternating Current Machines 3 by M. G. Say (1976), Electromagnetic Electromechanical Machines by Leander W. Matsch (1977), and Electrical Machines by A. Draper (1967). As is explained in such texts, the field coils producing magnetic flux in the alternator are disposed in the stator adjacent to the stator cores. The alternating current output windings are mounted on the stator cores near the air gaps which are between them and the rotor. As in any alternator, the path of least reluctance for the magnetic flux is through the rotor. The reluctance of the flux path, or magnetic circuit, bears a first order relation to the size of the air gaps between the stator and the rotor (provided, of course, that the magnetic flux density is not limited by magnetic saturation in some other part of the magnetic circuit). Usually, the surface of the rotor adjacent to the stator is provided with teeth, with gaps between them. (If desired, the gaps can be filled by a material with poor magnetic properties to provide a smooth surface on the rotor and reduce air friction or to increase the rotor's structural strength.) The path of least reluctance between the stator and the rotor, where most of the flux passes, is through the short air gaps between the stator and the rotor teeth. Much less flux passes through the larger air gaps in the slots between the rotor teeth where the reluctance is much greater. As the rotor rotates, its slots and teeth alternately come into positions opposite different sections of the stator cores, causing the magnetic flux through those sections to undergo periodic variations, thus indicating an emf in the output winding mounted there.
In some inductor alternators the stator, as well as the rotor, is provided with gaps and teeth. In these machines most of the flux passes through the stator and rotor teeth which are aligned with each other. Such toothed stator machines are used to produce high frequency output without necessitating closely spaced output windings. The pitch of the output winding mounted on the stator cores of other inductor alternators is matched with the rotor teeth.
Inductor alternators have been used to generate high frequency power where the structural limitations, or the rotational speed limits (often resulting from brushes and sliprings) render traditional alternators impractical. Inductor alternators are also used with drivers having very high shaft speeds (such as gas turbines) where traditional non-inductor alternators would require heavy and expensive reduction gearing. Traditional non-inductor alternators have usually been preferred over inductor alternators for low and moderate speed applications despite the disadvantage of brushes and sliprings. Unlike traditional non-inductor alternators, the flux through the output windings in an inductor alternator does not change direction but rather goes from some minimum to a maximum of the same polarity. Thus, all things being equal, an inductor alternator has roughly half the output emf of a similarly sized traditional alternator operating at the same shaft speed.
This invention provides a variety of inductor alternators that can replace non-inductor alternators in low and moderate speed applications. Conventional ways of replacing a non-inductor alternator in such low and moderate speed applications include increasing the size of the inductor alternator, or increasing its shaft speed to roughly twice that of a comparable non-inductor alternator. The former solution has the disadvantage of increased size and cost while the latter solution usually introduced difficulties with bearings and step-up gearing. This invention overcomes both of these disadvantages.